The distance measuring device representing a recent surveying instrument emits a measuring wave such as laser beams or microwaves to a distance measuring object, detects a reflected wave from the distance measuring object (hereinafter, generally named as a measuring wave), calculates a to-and-fro travel distance of the measuring wave on the basis of a time difference between an emission time and a detection time of the measuring wave, and thereby calculates the distance to the distance measuring object.
Here, the measurement of a time difference (time interval) has been performed, for example, by generating a high frequency clock signal having an extremely short already-known cycle, in comparison to a time difference between an emission time of a measuring wave and a detection time thereof, counting the clock number of the high frequency clock signal which are generated between the emission time and the detection time of the measuring wave, and multiplying the counted number by the cycle. However this measurement needs to raise the frequency of the clock signal in order to enhance the measurement accuracy; and yet, it embraces a limitation of raising the frequency of the clock signal.
Accordingly, a method has been developed which generates plural high frequency clock signals mutually having predetermined phase differences, counts the clock number of the clock signals each, and thereby brings the frequencies of the clock signals into high frequencies artificially. However, this method needs measurement repetitions corresponding to the number of the generated clock signals in enhancing the reliability of measurement accuracy, which makes the measurement time longer and invites a problem in practical use.
Accordingly, the present applicant has proposed a technique which generates a start signal synchronized with an emission time of a measuring wave and a stop signal synchronized with a detection time thereof repeatedly plural times with predetermined time intervals each, samples a reference signal of a sine wave or the like generated in a shorter cycle than this repetition cycle with a repetitive start signal, samples it with a repetitive stop signal, calculates a phase difference of a first sampling wave acquired by the sampling with the start signal and a second sampling wave acquired by the sampling with the stop signal, calculates a phase difference between the start signal and the stop signal based on this phase difference, and converts the acquired phase difference into a time difference (PATENT DOCUMENT 1).    PATENT DOCUMENT 1: Japanese Patent Registration No. 2916780 Bulletin